Liver X Receptors (LXR) as Therapeutic Targets in Dyslipidemia

Abstract

Liver X receptors (LXR) alpha and beta belong to a family of nuclear receptors which form heterodimers with the retinoid X receptor (RXR) and, upon ligand binding, stimulate the expression of target genes. LXR were initially described as orphan receptors and later oxidized cholesterol derivatives (oxysterols) were identified as their natural ligands. In addition, several synthetic LXR agonists such as T0901317 and GW3965 were synthesized. Oxysterols are formed in amounts proportional to cholesterol content in the cell and therefore LXR operate as cholesterol sensors which protect from cholesterol overload by inhibiting intestinal cholesterol absorption, stimulating cholesterol efflux from cells to high-density lipoproteins (HDL), its transport to the liver, conversion to bile acids, and biliary excretion. In addition, LXR agonists activate fatty acid synthesis by stimulating the expression of a lipogenic transcription factor, sterol regulatory element-binding protein-1c (SREBP-1c), leading to the elevation of plasma triglycerides and liver steatosis. Lipogenic effect seems is the most important negative feature of LXR agonists considered as potential hypolipidemic drugs. Some of currently used drugs also affect LXR signaling. For example, statins may impair LXR signaling by inhibiting oxysterol synthesis, whereas fibrates and thiazolidinediones increase LXR expression and activity.